As the performance of a CPU (Central Processing Unit) mounted on a board advances, the number of layers of the board increases and the thickness of the board may become thick. When the thickness of the board becomes thick, there arises a problem of difficulty in inserting a card edge of the board into a connector.
One of the methods for solving such a problem is forming a board into a stepped structure. A technique relating to such a board having a stepped structure is described in, for example, Patent Document 1. Patent Document 1 describes a printed wiring board having a stepped structure. According to Patent Document 1, the printed wiring board having the stepped structure has one or more noise reduction shield holes or heat dissipation holes formed between a signal line and a signal line adjacent thereto. Moreover, the holes are connected to a ground layer formed of a thick metal foil or a thin metal plate.
Further, a related technique is described in, for example, Patent Document 2. Patent Document 2 describes a flexible board which includes a contact part, a reinforcing plate reinforcing a face opposite to a face where a conductive pattern of the contact part is exposed, and a through hole.
Further, a related technique is described in Patent Document 3, for example. Patent Document 3 describes a connector circuit board which has a rigid board that the cross section of its end portion has a curved shape and a flexible board covering the curved face.
Patent Document 1: Japanese Unexamined Patent Application Publication No. JP-A 2013-115110
Patent Document 2: Japanese Unexamined Patent Application Publication No. JP-A 2000-196210
Patent Document 3: Japanese Unexamined Patent Application Publication No. JP-A 2010-062350
A board having a stepped structure as described in Patent Document 1 is manufactured by, for example, thinning a thick board with a drill or the like. In a case where a stepped structure is manufactured by thus thinning, there is a fear that a terminal of the board on the thinned face side becomes flat with a portion other than the terminal of the board, and contact failure may occur, that is, a terminal on the connector side comes in contact with the portion other than the terminal of the board.
Thus, there was a problem of difficulty in reducing a possibility of occurrence of contact failure in a board having a stepped structure. Even by using the techniques described in Patent Documents 2 and 3, it was impossible to solve such a problem.